The relevance of this study is due, first of all, to the absence of works devoted to the history of the Sumy Theological School in the national historiography. It is indicated that the main obstacle to carrying out research on the topic is the lack of archival documents. The main group of sources of the article, on which the authors relied, are the publications of the journals of the meetings of the clergy of the Sumy school district, reports, lists of pupils, messages and appeals related to the life of the school, published in the periodicals of the diocese – «Letter for the Kharkiv Diocese», «Vedomosti and notes on the Kharkiv Diocese». These materials made it possible to recreate a more or less complete picture of school life and realize the goal of the work – to reveal the most significant aspects of the functioning of the theological school in the city of Sumy at the end of the 19th – at the beginning of the 20th century. The main text of the publication contains the following parts: a description of the circumstances of the foundation of the Sumy Theological School; description of the educational institution's finances (estimate, sources of replenishment); management of the school (congress of clergy of the Sumy school district, school board, school supervisor, custodian); teaching staff of the school; organization of educational work (educational disciplines, academic performance, Christian upbringing, cultural and educational events with the participation of students); closure of the educational institution. The authors emphasized the important role in the emergence of the educational institution of the Sumy deacon Fr. Vasyl Nikolskyi, Archbishop of Kharkiv and Okhtyrskyi Amvrosii (Klyucharyov), Sumy businessmen I. G. Kharitonenko and D. I. Sukhanov. Among the staff of the school, more attention was paid to its supervisor A. F. Gruzov and teacher F. F. Gorain according to the availability of sources. The conclusions indicate the prospects for further research on the chosen topic. First of all, it is an interesting reproduction of the psychological atmosphere of the school, the nuances of the teachers' pedagogical creativity, the relationships within the student groups, as well as between students and teachers. The authors consider their research in the field of microhistory as a component of a complex of research on the history of secondary education institutions in Sumy.